Problem 62 - Chemistry The Central Science In Si Units

Question

Write a balanced equation for each of the following reactions: (a) hydrolysis of \(\mathrm{PCl}_{5},\) (b) dehydration of phosphoric acid (also called orthophosphoric acid) to form pyrophosphoric acid, \((\mathbf{c})\) reaction of \(\mathrm{P}_{4} \mathrm{O}_{10}\) with water.

Step-by-Step Solution

Answer

(a) \( \mathrm{PCl}_{5} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{POCl}_{3} + \mathrm{2HCl} \); (b) \( 2 \mathrm{H}_3\mathrm{PO}_4 \rightarrow \mathrm{H}_4\mathrm{P}_2\mathrm{O}_7 + \mathrm{H}_2\mathrm{O} \); (c) \( \mathrm{P}_4\mathrm{O}_{10} + 6 \mathrm{H}_2\mathrm{O} \rightarrow 4 \mathrm{H}_3\mathrm{PO}_4 \).

Step 1: Understand the reaction types

For part (a), hydrolysis means breaking down a compound with water. For part (b), dehydration involves the removal of water molecules. In part (c), reacting a compound with water usually involves hydration.

Step 2: Balance hydrolysis of \( \mathrm{PCl}_{5} \)

Identify the reaction: \( \mathrm{PCl}_{5} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{POCl}_{3} + \mathrm{2HCl} \). The phosphorus pentachloride reacts with water to form phosphorus oxychloride and hydrochloric acid.

Step 3: Balance dehydration of phosphoric acid

The balanced equation is: \( 2 \mathrm{H}_3\mathrm{PO}_4 \rightarrow \mathrm{H}_4\mathrm{P}_2\mathrm{O}_7 + \mathrm{H}_2\mathrm{O} \). Two molecules of orthophosphoric acid lose water to form one molecule of pyrophosphoric acid.

Step 4: Balance reaction of \( \mathrm{P}_{4} \mathrm{O}_{10} \) with water

The balanced equation is: \( \mathrm{P}_4\mathrm{O}_{10} + 6 \mathrm{H}_2\mathrm{O} \rightarrow 4 \mathrm{H}_3\mathrm{PO}_4 \). The tetraphosphorus decoxide reacts with water to produce four molecules of phosphoric acid.

Key Concepts

Hydrolysis Reaction

Hydrolysis reactions involve the splitting of a compound due to a reaction with water. In this process, water molecules break bonds within another substance, resulting in the formation of two or more new substances. A classic example of a hydrolysis reaction is when phosphorus pentachloride (\(\mathrm{PCl}_5\)) reacts with water. Here, the water molecule attacks the \(\mathrm{PCl}_5\), breaking it down into phosphorus oxychloride (\(\mathrm{POCl}_3\)) and hydrochloric acid (\(\mathrm{HCl}\)). This can be expressed with the balanced equation: \[ \mathrm{PCl}_{5} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{POCl}_{3} + 2\mathrm{HCl}\]

• \(\mathrm{PCl}_5\) is broken down by water's interaction.
• The water's hydrogen and oxygen atoms rearrange to form new compounds.
• Hydrolysis is crucial in various biological and chemical processes as it helps modify or breakdown substances.

Dehydration Reaction

Dehydration reactions, as the name suggests, involve the removal of water molecules from a compound. This type of reaction is common when forming larger biomolecules. In our example, orthophosphoric acid (\(\mathrm{H}_3\mathrm{PO}_4\)) undergoes dehydration to produce pyrophosphoric acid (\(\mathrm{H}_4\mathrm{P}_2\mathrm{O}_7\)). This removal of water can lead to the creation of more complex structures, which is represented by the balanced equation:\[2 \mathrm{H}_3\mathrm{PO}_4 \rightarrow \mathrm{H}_4\mathrm{P}_2\mathrm{O}_7 + \mathrm{H}_2\mathrm{O} \]

• Two molecules of orthophosphoric acid lose one water molecule.
• This process helps in the formation of long chain molecules in nature.
• Dehydration is often reversible, where adding water can regenerate the original compound.

Phosphoric Acid Reactions

Phosphoric acid, a versatile chemical, participates in several reactions. A prominent reaction is its formation through the hydration of tetraphosphorus decoxide (\(\mathrm{P}_4\mathrm{O}_{10}\)) with water. The balanced chemical equation is as follows:\[\mathrm{P}_4\mathrm{O}_{10} + 6 \mathrm{H}_2\mathrm{O} \rightarrow 4 \mathrm{H}_3\mathrm{PO}_4\]Here, water molecules react with \(\mathrm{P}_4\mathrm{O}_{10}\) to yield four molecules of phosphoric acid. This reaction is a prime example of a type of hydration reaction found in phosphoric compounds.

• It highlights the transformation of phosphorus compounds in the presence of water.
• The reaction is commonly seen in the industrial preparation of phosphoric acid.
• Phosphoric acid produced is widely used in fertilizers, beverages, and cleaning agents.